Cruise CG
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Cruise CG
On the Perf Init page the default setting for cruise CG is 18.5% for the B737-300.
What is your company procedure, do you alter this or just leave it.
Also noticed that on the B737-300 winglet equiped AC the default setting is 4.0%
Is this the same on other winglet equiped AC for other operators? Seems like quite a difference!!
Altering the cruise CG only makes a few hundred feet difference between the Max ALT anyway. But just wondering why there is so much difference.
Any feeback greatly appreciated.
What is your company procedure, do you alter this or just leave it.
Also noticed that on the B737-300 winglet equiped AC the default setting is 4.0%
Is this the same on other winglet equiped AC for other operators? Seems like quite a difference!!
Altering the cruise CG only makes a few hundred feet difference between the Max ALT anyway. But just wondering why there is so much difference.
Any feeback greatly appreciated.
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I am agin the idea of default
I’m a little worried by this.
The cg position determines the amount of “trim”, the amount of “trim” has an effect on drag and in conjunction with Cost Index I would expect to see some influence on optimum cruise level. But the proper reading of cg position is to understand how it relates to longitudinal stability in the first instance. Stability first, cruise second. And if an FMS wakes up with a figure of 4% when you first hit Perf Init, don’t believe it. You won’t beat the Wright brothers first attempt at flight, you won’t reach 35ft and the resulting fuel burn will be anything but economic. I'd take an EWAG that the majority of aircraft are at their happiest 25%mac plus or minus 10%. And I always liked to enter the figure as determined by the proper method. I rather liked the 737-400 by the way and the 767-300 too but they didn't have retro winglets in my day.
The cg position determines the amount of “trim”, the amount of “trim” has an effect on drag and in conjunction with Cost Index I would expect to see some influence on optimum cruise level. But the proper reading of cg position is to understand how it relates to longitudinal stability in the first instance. Stability first, cruise second. And if an FMS wakes up with a figure of 4% when you first hit Perf Init, don’t believe it. You won’t beat the Wright brothers first attempt at flight, you won’t reach 35ft and the resulting fuel burn will be anything but economic. I'd take an EWAG that the majority of aircraft are at their happiest 25%mac plus or minus 10%. And I always liked to enter the figure as determined by the proper method. I rather liked the 737-400 by the way and the 767-300 too but they didn't have retro winglets in my day.
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From memory the FAA (USA) and CAA (or JAA Europe) had a different certification philosophy with regard to FMC predicted max altitudes and cruise c of g. The Americans used best case c of g (highest alt capability) and 1.2g buffet onset. The Europeans used statistically normal case c of g and 1.3 g. When it became technically possible to automatically update cruise c of g in the FMC the Americans refused, because they lost apparent capability. So for a long time it was only possible to enter the c of g manually into the FMC - that may still be the case, I don't know. Typical in service figures would range from about 11 to about 24 depending on a/c type etc.
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enicalyth,
This is not about flying at 4% CofG, it is about selecting 4% in the cruise page of the FMC. Getting to 35' or not has little to do with the cruise CofG selected in the FMC.
We leave the cruise CofG at the default 5% (wingletted 738). This gives the most conservative presentation of maximum cruise level, but has no influence on optimum cruise level. Just provides an extra bit of protection.
This is not about flying at 4% CofG, it is about selecting 4% in the cruise page of the FMC. Getting to 35' or not has little to do with the cruise CofG selected in the FMC.
We leave the cruise CofG at the default 5% (wingletted 738). This gives the most conservative presentation of maximum cruise level, but has no influence on optimum cruise level. Just provides an extra bit of protection.
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Does anyone know the approximate down load on the tail of a B737-300 at mid cg as a percentage of weight and the variation that occurrs between cg limits. The smaller the tail down load the smaller the total drag but know that the cg limits are at the edges of acceptable stability and often controllability.
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"Does anyone know the approximate down load on the tail of a B737-300 at mid cg as a percentage of weight and the variation that occurrs between cg limits."
What download (MAC) is assumed for optimum level calcs by the fmc?
What download (MAC) is assumed for optimum level calcs by the fmc?
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From what I can see from the Boeing documentation, the difference in default CG (4% FAA - 18.5% JAA), approximates a +/- 2-tonne (!) difference in aircraft mass when the FMS calculates max FL. (the FMC adjusts total a/c mass to compensate for more/less tail down force). Hence, we always take the load sheet take-off CG and subtract 4%, then put this number into the FMC. Usually ends up around 12-16% on our B737-300.
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This thread seems to have opened up a "can of worms"!
Downloads by the tails significantly decrease the ease of an aircraft's progress though the air. The value of the download varies considerably to preserve natural stability over the speed (IAS and Mach No) cleared envelope and as cg position changes.There are a few other minor variables.
Do I detect that predicted tail downloads are sometimes being used as a variable in performance expectations by the addition of the download to the aircraft mass to produce an apparentl weight?
It's a while since I have been active as a TP but in my day when I did a measured take off and other performance tests I always assumed that the aircraft designer had taken into account the tail down loads when calculating the aircraft's weight being supported by the wings and any body lift.
The onset of acceptable artificial stability and an increasing emphasis on aircraft efficiency must be impelling us to accept yet another variable in flying with minimum drag.
Downloads by the tails significantly decrease the ease of an aircraft's progress though the air. The value of the download varies considerably to preserve natural stability over the speed (IAS and Mach No) cleared envelope and as cg position changes.There are a few other minor variables.
Do I detect that predicted tail downloads are sometimes being used as a variable in performance expectations by the addition of the download to the aircraft mass to produce an apparentl weight?
It's a while since I have been active as a TP but in my day when I did a measured take off and other performance tests I always assumed that the aircraft designer had taken into account the tail down loads when calculating the aircraft's weight being supported by the wings and any body lift.
The onset of acceptable artificial stability and an increasing emphasis on aircraft efficiency must be impelling us to accept yet another variable in flying with minimum drag.
